U.S. Pat. No. 4,691,843 and WO 90/08724 disclose discharge arrangements comprising a piston-cylinder assembly. However, such a piston-cylinder assembly tends to jam when the bulk material presses asymmetrically onto the closure member. In fact, the piston is guided in the known arrangements by the outer circumference of the piston within the cylinder, on the one hand, and by a bushing in one front end of the cylinder through which passes the piston rod, on the other hand. Since the distance between these two guides is smallest as soon as the piston reaches the position which corresponds to the open position of the closure member, the stability of the guidance is poor just when the closure member opens and is vibrated either by the outflowing bulk material or a vibrator for promoting flow of material or both. Therefore, the forces acting transversely to the axis of the piston and cylinder are strongest in a position where the guidance is weak.
A further disadvantage of the known arrangements is that the guiding zones exert high frictional forces obstructing the piston's movement. This is particularly disadvantageous for various purposes envisaged. For example, receptacles of the above-mentioned kind are often bins used to fill so-called Intermediate Bulk Containers or IBC's used to transfer hazardous or sensitive bulk material. When the IBC has been filled up, the discharge opening of the bin should be closed as quickly as possible to avoid an overflow. Therefore, it was necessary, heretofore, to use piston-cylinder arrangements which are actuable from both sides to effect opening and closing movement. This involves higher expenditure for establishing a corresponding two-sided control.
U.S. Pat. No. 4,470,524 discloses the use of an inflatable bellow to avoid the risk of jamming, thus dispensing with a rigid guidance. This entails more flexibility with respect to the vibrations mentioned above. By supplying a fluid under pressure, the bellow is expanded substantially in a longitudinal direction, thus opening the closure member. When an IBC is filled up, pressurization is interrupted so that the bellow is compressed by the weight of the parts above it, and especially by the weight of the bulk material. Since a bellow is, however, also expandable in a direction transverse to the longitudinal direction, a relatively large amount of pressurized air is required and, consequently, a large amount of time is required to press it out again. Therefore, the closure time attained by such a construction, from the moment where closure is commanded up to complete shut off of the discharge opening, is too long as to ensure exact metering of the bulk material. Slow closure is also caused by a relatively high inherent deformation resistance of the bellows.
Although a linear guiding unit has been assigned to the bellow, its high flexibility leads to a tendency to digress from a straight path of movement whenever the bulk material presses asymmetrically onto the closure member. If the bulk material contains agglomerated particles, these larger particles can be clamped in the annular space between the closure member and the discharge opening in an asymmetric position, thus preventing complete closure of the opening. Thus, although flexibility is advantageous under vibrations in an open position, it is most disadvantageous for closing the discharge opening completely.